Generally, in marine vessels having little or no cargo, ballast water is introduced into a ballast tank before the departure of these marine vessels from the necessity of securing the submerged depth of a propeller and ensuring safe navigation on voyage with no cargo. When cargo is loaded in the harbor on the contrary, ballast water is discharged. In the meantime, if ballast water is taken on by and discharged from a marine vessel which plies between a loading port and an unloading port differing in environment, a difference in the type of bacteria and microorganisms such as plankton contained in ballast water arouses a fear as to an undesirable influence on the coastal ecosystem. For this reason, in the international conference held in February, 2004 with reference to the control of ballast water in marine vessels, an international treaty on the limit and control of ballast water and deposits from marine vessels was eventually adopted and imposed an obligation to treat ballast water.
According to the standard established by the International Maritime Organization (IMO) as the standard for treating ballast water, the number of microorganisms (mainly, zooplankton) 50 μm or larger in size which are contained in ballast water discharged from a marine vessel is less than 10 in 1 m3, the number of microorganisms (mainly phytoplankton) 10 μm or more and less than 50 μm in size is less than 10 in 1 mL, the number of Vibrio cholerae is less than 1 cfu in 100 mL, the number of colibacilli is less than 250 cfu in 100 mL and the number of enterococci is less than 100 cfu in 100 mL.
Technologies used to treat ballast water are still in progress in many institutions concerned. In prior technologies, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-181443 discloses an apparatus used to sterilize ballast water by utilizing high-temperature exhaust gas emitted from the main engine of a marine vessel. Also, Jpn. Pat. Appln. KOKAI Publication Nos. 4-322788 and 5-000910 disclose a method of sterilizing harmful plankton or cysts of harmful algae contained in ballast water by using a chlorine-based bactericide or hydrogen peroxide when the ballast water is discharged. Particularly, Jpn. Pat. Appln. KOKAI Publication No. 4-322788 discloses a process in which air is blown into ballast water by an aerating apparatus when the ballast water is discharged to make harmless chlorine (residual chlorine) left after cysts of harmful algae are destroyed.
However, the number of organisms contained in sea water to be used as ballast largely differs depending on the date and time when and place where the sea water is sampled and widely varies between about a few organisms and about hundreds of millions of organisms in 1 mL of sea water. The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-181443 has a difficulty in exactly achieving all requirements of the standard established by the International Maritime Organization (IMO). Also, the methods described in Jpn. Pat. Appln. KOKAI Publication Nos. 4-322788 and 5-000910 have the problems that these methods have no effect on relatively large zooplankton and microorganisms resistant to bactericides and that the influence of remaining bactericide on the environment when the bactericide is discharged together with the ballast water cannot be neglected.
Even if a chlorine-based bactericide is added to ballast water to destroy cysts of harmful algae in the method of adding a chlorine-based bactericide among the methods described in Jpn. Pat. Appln. KOKAI Publication No. 4-322788, residual chlorine afterwards reacts with organic materials in ballast water to generate toxic trihalomethanes. However, in this publication, nothing is considered about this trihalomethanes. Even if air is blown into ballast water by using an aeration apparatus, only a part of these trihalomethanes transfer to a vapor phase but a large part are left unremoved in a liquid phase and are not made harmless. Then, ballast water including these trihalomethanes is discharged and exerts unfavorable influences on the environment. On the other hand, the method in which hydrogen peroxide is added among the methods described in this publication has the problem that it is difficult to destroy bacteria in ballast water by hydrogen peroxide to the extent that the standard of ballast water treatment of the IMO is satisfied.